Constant velocity elastomeric joint

ABSTRACT

A constant velocity joint (10) is provided specifically for connecting a yoke (14) to a mast (12) in a helicopter and more generally (200) for interconnecting a first shaft (202) to a second shaft (204) for bidirectional drive. The constant velocity joint employs a carrier (40, 218) which is formed with a series of apertures for receiving elastomeric bearing assemblies (42, 80, 220). The elastomeric bearing assemblies are provided with a spherical elastomeric bearing (60) and a cylindrical elastomeric bearing (48). Certain of the elastomeric bearing assemblies connect the carrier to one of the rotating members while the remaining elastomeric bearing assemblies connect the other rotating member to the carrier. The axis of symmetry (67) of the carrier always bisects the angle of misalignment between the two rotating shafts or members to assure that a constant velocity joint is achieved.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.794,800, filed Nov. 4, 1985 and entitled "Constant Velocity ElastomericJoint", now U.S. Pat. No. 4,729,753.

TECHNICAL FIELD

This invention relates to a constant velocity joint for connecting twononaligned members for joint rotation, and in particular to a constantvelocity joint between the mast and rotor hub of a helicopter.

BACKGROUND OF THE INVENTION

In power train design, it is often desirable and even necessary toconnect a driving member to a driven member for joint rotation when thetwo axis of rotation are misaligned. While the use of a simple Cardan(or Hooke) universal joint would connect the two for joint rotation,this joint will induce angular velocity variations in the driven member,causing vibration at a frequency of twice the rotational velocity andoscillation in the torque transferred to the driven member.

In an effort to provide for more uniform power transmission, constantvelocity joints have been developed. Frequently, such constant velocityjoints require some sliding motion between elements in the joint,generating friction and power loss. However, some designs, including theone disclosed in U.S. Pat. No. 4,208,889, employ a number of elastomericbearings to provide for movement in the constant velocity joint.

One specific application for the concept of the constant velocity jointis use in the drive train of a helicopter employing a flapping yoke. Insuch an environment, a mast connected to the power source rotates abouta fixed axis relative to the helicopter main frame. A hub assembly ismounted to the mast for joint rotation. The hub assembly includes a yokeand a torque transferring mechanism to transmit torque from the mast tothe yoke. The yoke is also supported by the torque transferringmechanism for flapping motion where the rotational axes of the mast andyoke can become misaligned. U.S. Pat. No. 4,323,332, issued Apr. 6,1982, discloses an effort to provide a flexible connection between amast and yoke. U.S. Pat. No. 4,477,225, issued Oct. 16, 1984, disclosesan attempt to provide an elastomeric mounting of a yoke to a mast with aconstant velocity joint.

In the particular environment of a helicopter, certain factors are ofcritical interest. The use of a constant velocity joint for the torquetransferring mechanism is advantageous in avoiding vibration that wouldbe induced by employing a conventional Cardan joint. In addition, thejoint should have a high torque transmitting capacity and preferablyrequire no lubrication. The joint should be self-centering in order toinfluence the yoke to return to alignment with the mast after flapping.If the joint should fail in service, the failure is preferably in anoncatastrophic failure mode which could permit the hub assembly toremain operational until the helicopter can be landed. It is also veryadvantageous to minimize the weight and size of the joint, therebyincreasing the useful payload of the helicopter and improving theaerodynamic configuration as well. Finally, some axial motion betweenthe yoke and mast is common and the joint should be capable oftransmitting torque despite this motion. In other environments, abidirectional drive feature, i.e. where either member could be thedriving member, can be advantageous.

At this time, no constant velocity joint has been developed whichsatisfies the above criteria to an adequate degree. Therefore, a needexists for development of a constant velocity joint which satisfiesthese requirements, and in particular for a constant velocity jointwhich is adaptable for use in a flapping yoke helicopter environment.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a constantvelocity joint is disclosed for joining a driving member to a drivenmember for joint rotation. The driving member rotates about a first axisand the driven member rotates about a second axis with the first andsecond axes being capable of misalignment. The constant velocity jointemploys a plurality of elastomeric bearing assemblies. Each bearingassembly has an inner rigid cylindrical section and an intermediaterigid section. A cylindrical elastomeric bearing is concentric with theinner and intermediate rigid sections and is bonded to the outercylindrical surface of the inner section and to the inner cylindricalsurface of the intermediate section. A spherical elastomeric bearing hasan inner spherical surface bonded to the outer spherical surface of theintermediate rigid section. A rigid carrier is provided which defines anaxis of symmetry and has a plurality of apertures formed therethroughcentered on axes parallel to and spaced from the axis of symmetry. Eachof the elastomeric bearing assemblies is positioned in one of theapertures, with the outer spherical surface of the spherical elastomericbearing being bonded to the inner spherical surface of the wall of theaperture. Structure is provided for rigidly securing the inner rigidcylindrical section of selected elastomeric bearing assemblies relativeto the driving member. Structure is provided for rigidly securing theinner rigid cylindrical sections of the other elastomeric bearingassemblies to the driven member. The cylindrical elastomeric bearingsdeform in an axial direction along the center axis of the bearing and ina direction generally perpendicular to the center axis and the first andsecond axis. The spherical elastomeric bearings deform angularly aboutthe center point of the spherical section. The deformation of theelastomeric bearings permit the driving member to rotate the drivenmember with constant velocity.

In accordance with another aspect of the present invention, the drivingmember is formed by the mast of a helicopter and the driven member isformed by the yoke of the helicopter. The structure for securingselected elastomeric bearing assemblies to the driving member is formedby a pair of spaced apart plates splined to the mast for rotationtherewith which secures the elastomeric bearing assemblies between theplates. The structure securing the other elastomeric bearing assembliesto the driven member comprise U-shaped pillow blocks which secure theother elastomeric bearing assemblies to the yoke.

In accordance with other aspects of the present invention, laminatedcylindrical and spherical elastomeric bearings can be employed toimprove fatigue life and reduce frictional wear. A flapping spring canalso be employed between the driving member and driven member to limitthe misalignment between the rotational axes of the members and totransfer axial forces or thrust between the members. A thrust linkagecan also be provided which transfers axial forces between the drivingand driven member.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention can be had by referringto the following Detailed Description together with the accompanyingdrawings, wherein:

FIG. 1 is a perspective view of a first embodiment of the presentinvention forming a constant velocity joint for a helicopter;

FIG. 2 is an exploded view of the constant velocity joint illustrated inFIG. 1;

FIG. 3 is a top view of the constant velocity joint of FIG. 1 partiallybroken away to better illustrate the carrier;

FIG. 4 is a vertical cross-sectional view of the constant velocity jointillustrated in FIG. 1 taken along multiple planes to show an elastomericbearing assembly between the mast and carrier and an elastomeric bearingassembly between the carrier and the hub;

FIG. 5 illustrates the same vertical cross-sectional view shown in FIG.4 showing a misalignment between the rotational axes of the mast andyoke;

FIGS. 6a and 6b are schematic drawings of the constant velocity jointillustrated in FIG. 1 with the elastomeric bearing assemblies removed tobetter illustrate the plates splined to the rotor mast, the carrier andthe pillow blocks secured to the yoke;

FIG. 6c is an exploded view of an elastomeric bearing assembly;

FIG. 6d is a partial cross section of a modified elastomeric bearingassembly having laminated elastomeric bearings;

FIG. 7 is a vertical cross-sectional view of a constant velocity jointforming a second embodiment of the present invention intended for use inconnecting any driving and driven members with constant velocity,including a thrust linkage for transferring axial loads between therotating members;

FIG. 8 is an end view of the constant velocity joint illustrated in FIG.7; and

FIG. 9 is a cross-sectional view of the constant velocity jointillustrated in FIG. 7 showing a misalignment between the axes ofrotation of the members.

DETAILED DESCRIPTION

Referring now to the drawings, wherein like reference numerals designatelike or corresponding elements throughout the views, there is shown inFIGS. 1-6 a constant velocity joint 10, which comprises a firstembodiment of the present invention. The joint 10 is employed totransfer torque from the mast 12 in a helicopter to a yoke 14 forrotating the yoke and blades attached thereto to lift the helicopter.The yoke 14 is supported through the constant velocity joint 10 forflapping motion, where the axis of rotation of the yoke 14 can becomemisaligned with the axis of rotation of the mast 12. Despite suchmisalignment, the constant velocity joint 10 will provide for a smoothtransfer of torque from the mast to the yoke without inducing vibration.Together, joint 10 and yoke 14 form the hub assembly of a helicopter.

With particular reference to FIG. 2, the mast 12 can be seen to havesplined sections 16 and 18 along its length, and a threaded end 20. Themast 12 is supported by the helicopter main frame and rotated about thedriving axis 22 by the power plant in the helicopter. A lower triangularplate 24, having inner splines 26, is slid over the end of the mast 12for engagement with spline section 16 for rotation with the mast. Anupper triangular plate 28, having inner splines 30, is similarly splinedto section 18. The plates 24 and 28 are axially restrained along axis 22between shoulder 104 on mast 12 and a nut 106 threaded on threads 20.Each of the three arms 32 of the plates 24 and 28 extend radiallyoutward from the driving axis and end with a hole 34. Through bolts 36connect the two plates 24 and 28, as best seen in FIG. 6a.

A carrier 40 is supported between the plates 24 and 28 and by the platesthrough three elastomeric bearing assemblies 42. With reference to FIGS.4 and 6c, each elastomeric bearing assembly 42 can be seen to comprise arigid inner cylindrical section 44 rigidly secured between the plates 24and 28 by through bolt 36. The center elongate axis 46 of thecylindrical section 44 is thus parallel to, but radially spaced from,the driving axis 22. A tapered cylindrical elastomeric bearing 48 isprovided and is concentric with the cylindrical section 44. The innercylindrical surface 50 of the elastomeric bearing 48 is bonded orotherwise secured to the outer cylindrical surface 52 of the cylindricalsection 44. As an alternative, surface 50 can simply be secured directlyto bolt 36, although use of section 44 is preferred for ease ofreplacing assemblies 42 when worn or failed.

A rigid intermediate section 54 is concentric with section 44 andbearing 48 when the bearing assembly is not subjected to externalforces, and the outer cylindrical surface 56 of the elastomeric bearing48 is bonded or otherwise secured to the inner cylindrical surface 57 ofthe intermediate section 54.

A spherical elastomeric bearing 60 has an inner spherical surface 62which is bonded or otherwise secured to an outer spherical surface 63 ofintermediate section 54.

With reference to FIGS. 6a and 6b, the carrier 40 can be seen tocomprise an annular ring having a series of six apertures 66 formedtherethrough around its circumference. The apertures 66 are generallycentered along axes parallel to the axis of symmetry 67 of the carrier40 and at a uniform radius from the axis of symmetry. The elastomericbearing assemblies 42 are received in three of the apertures 66 in asymmetrical pattern on the carrier 40. The outer spherical surfaces 70of spherical elastomeric bearings 60 (see FIG. 6c) are bonded orotherwise secured to the walls 72 of the apertures 66 and the walls 72are preferably also configured as a spherical surface of equal radius tothe outer spherical surfaces 70.

It can thus be seen that the three elastomeric bearing assemblies 42support the carrier 40 between the plates 24 and 28 fixed to the mast12. However, the elastomeric bearings 48 and 60 within each elastomericbearing assembly are capable of deformation to permit the axis ofsymmetry 67 of the carrier 40 to become misaligned with the driving axis22. In particular, the configuration and design of the elastomericbearings 48 and 60 provide for three spring rates per elastomericbearing assembly 42. With particular reference to FIG. 6c, K₁ representsthe combined axial spring rates of the cylindrical elastomeric bearings48 and 60 for deformation along the center axis 46. The value K2represents the radial spring rate of the elastomeric bearings 48 and 60for motion perpendicular to the center axis 46. The value K₃ representsthe angular spring rate of the elastomeric bearings 48 and 60 about thecommon center 74 of the elastomeric bearing 60, surface 54 and wall 72,which lies on the center axis 46. If desired, each elastomeric bearingcan be laminated, as shown in FIG. 6d, with multiple layers ofelastomeric material separated by rigid sections 49 in bearing 48' andrigid sections 61 in bearing 60'. The sections 49 and 61 are preferablycontoured to the shape of the bearings. The sections 48 are thereforegenerally cylindrical while the sections 61 are formed with sphericalsurfaces.

With reference to FIG. 2, three elastomeric bearing assemblies 80 can beseen to secure the hub 14 to the carrier 40 for joint rotation, yetpermit angular misalignment between the driven axis 82 of the hub 14(see FIG. 5) and the axis of symmetry 67 of the carrier 40. Theelastomeric bearing assemblies 80 are in all respects identical to theelastomeric bearing assemblies 42 and are interchangeable therewith.Each of the elastomeric bearing assemblies 80 is secured to the hub by abolt 84 secured to the hub and supported at its upper end by a pillowblock 86 having a U-shaped configuration. Each pillow block 86 isrigidly secured to the hub 14 by bolts 87 as best seen in FIG. 6b.

Torque transmission to the yoke 14 from mast 12 is transmitted throughelastomeric bearing assemblies 42, carrier 40 and elastomeric bearingassemblies 80. Thrust (or rotor lift) is transmitted from yoke 14 tomast 12 through elastomeric flapping springs 116 and 118 discussedhereinafter. Therefore, the yoke 14 is supported by the mast 12 throughbearing assemblies 42, carrier 40 and bearing assemblies 80 without anonelastomeric connection. This permits the yoke 14 to flap relative tothe mast 12 about a flapping center 88 (see FIG. 5), so that the joint10 will permit the driving axis 22 and driven axis 82 to becomemisaligned by an angle α, as best seen in FIG. 5. The configuration ofthe constant velocity joint 10 is such that the axis of symmetry 67 ofthe carrier 40 will be misaligned with both the driving axis 22 anddrive axis 82 by an angle α/2. The axis of symmetry 67 will always bepositioned to exactly bisect the angular misalignment between the mastand hub and assures that the joint will have constant velocity.

It will be observed that the flapping center 88 generally lies along theaxis of symmetry of carrier 40 at the midpoint through the thickness ofthe carrier 40. The center 74 of each of the elastomeric bearingassemblies 42 and 80 are also preferably in the plane that isperpendicular to the axis of symmetry 67 and contains the midpointthrough the thickness of carrier 40. This provides a compact arrangementfor joint 10.

Due to the stiffness of the elastomeric bearings, the misalignment ofthe driving and driven shafts will create forces in the bearings urgingthe mast and hub back into alignment. These forces contribute to thecontrol moment applied at the flapping center 88 of the yoke 14, whichis defined as the point about which the yoke pivots relative to the mast12.

The moment quantity contributed by the constant velocity joint 10 iscalculated as follows:

    M=1.5 r.sup.2 sin α/2 [K.sub.1 +(1-cosα/2) K.sub.2 ]+3 (α/2) K.sub.3

K₁ =the axial spring rate of one elastomeric bearing assembly (lb/in).

K₂ =radial spring rate of one elastomeric bearing assembly (lb/in).

K₃ =the angular spring rate of one elastomeric bearing assembly(in-lb/degree).

r=coupling radius at bearings (in).

α=misalignment between yoke 14 and rotor mast 12 (degrees).

In one constant velocity joint constructed in accordance with theteachings of the present invention, a limit torque of 967,500 inchpounds was assumed. The springs rates for the elastomeric bearingassemblies would be as follows:

K₁ =4830 lb/in.

K₂ =340,000 lb/in.

K₃ =676 in-lb/deg.

While joint 10 can be used as a hub assembly drive joint as describedwithout use of elastomeric bearings 116 and 118, as long as plates 24and 28 are confined along axis 22, preferably the constant velocityjoint 10 will have structure for limiting the angular misalignment andcarrying the rotor thrust between the driving axis 22 and the drivenaxis 82. Therefore, a flapping and axial load transfer spring is mountedon joint 10. In particular, a lower ring 100 can be splined to splinesection 16 for rotation with the mast 12 as best seen in FIGS. 4 and 5.An upper ring 102 can also be splined to the spline section 18 forrotation with mast 12. Both rings 100 and 102 and plates 24 and 28 canbe secured on the mast 12 between the shoulder 104 and nut 106 threadedon end 20 of the mast 12. Both lower and upper rings 100 and 102 definespherical surfaces 108 which are centered on the flapping center 88.

A lower bowl 110 is secured to the yoke 14 by the bolts 87 that securethe pillow blocks 86 to the yoke 14. An upper bowl 112 is secured to theyoke 14 above the pillow blocks 86 by the bolts 87. Both lower bowl 110and upper bowl 112 can be seen to have spherical surfaces 114 alsocentered on flapping center 88.

A lower spherical elastomeric bearing 116 is bonded or otherwise securedto the spherical surfaces of lower ring 100 and lower bowl 110. An upperspherical elastomeric bearing 118 is bonded or otherwise secured betweenthe spherical surfaces on upper ring 102 and upper bowl 112. It can thusbe seen that the angular misalignment permitted between the axes 22 and82 is limited by the deformation in compression of the elastomericbearings 116 and 118 and that the rotor thrust is carried by bearings116 and 118.

With reference now to FIGS. 7-9, a second embodiment of the presentinvention is illustrated, forming a constant velocity joint 200. Thejoint 200 connects a first shaft 202 with a second shaft 204 for commonrotation even if the rotational shaft 206 of the first shaft 202 ismisaligned with the rotational axis 208 of the second shaft 204. It willalso be understood that either of the shafts can be the driving ordriven member, as the constant velocity joint 200 is bidirectional.

The first member will be seen to have a triangularly shaped plate 210secured at the end thereof. Second shaft 204 will be seen to have asimilar triangular shaped plate 212 fixed at its end. Each of the arms214 of plates 210 and 212 extend radially outward from the axis ofrotation of the shaft to receive a bolt 222.

A carrier 218, identical in function to carrier 40 and having an axis ofsymmetry 219, is positioned between the ends of shafts 202 and 204 andalso supports six elastomeric bearing assemblies 220 in the identicalmanner as carrier 40 supports elastomeric bearing assemblies 42 and 80.Elastomeric bearing assemblies 220, in turn, are identical in structureand function with the elastomeric bearing assemblies 42 and 80.Alternating bearing assemblies 220 about the circumference of thecarrier 218 are secured through their inner cylindrical sections rigidlyto an arm 214 of plate 210 or 212 by bolts 222. Torque can thus betransmitted from one shaft to the other through the elastomeric bearingassemblies 220 and carrier 218 in a manner substantially identical toconstant velocity joint 10. As with carrier 40, the configuration ofconstant velocity joint 200 will always have the axis of symmetry 219 ofthe carrier 218 at an angle bisecting the angle of misalignment betweenthe axes 206 and 208 as best seen in FIG. 9.

In addition, constant velocity joint 200 includes a thrust linkage 224.The thrust linkage 224 includes a cleavis 226 formed on first shaft 206having two parallel legs 228. A spherical ball 230 is secured betweenlegs 228 by pin 232. An elastomeric bearing 234 having the shape of aspherical segment is bonded or otherwise secured on its inner surface tothe ball 230. The outer surface is bonded or otherwise secured to theinner surface of a ring 236 extending from the first shaft. The thrustlinkage 224 therefore permits the transfer of thrust forces betweenshafts 202 and 204 along their rotational axes while permitting somemisalignment between the rotational axes.

A constant velocity joint has thus been disclosed which has significantadvantages over prior art designs. The joint can be used in theenvironment of a flapping helicopter hub assembly, but can also be usedwith any power train, even with bidirectional torque transfer. The jointeliminates vibration common with nonconstant velocity joints.Furthermore, it requires no lubrication, is selfcentering andaccommodates limited axial motion between the rotating members. Thejoint has a noncatastrophic failure mode should the elastomeric bearingsshear or separate, which can be highly advantageous in environments suchas a helicopter hub assembly. The placement of the elastomeric bearingassemblies in a carrier in a single plane minimizes the size of thejoint and provides for most effective transfer of torque. A high torquetransfer capacity is achieved, both by the absence of sliding frictionbetween elements in the joint and the absence of severe shear in theelastomeric bearings.

Although several embodiments of the invention have been illustrated inthe accompanying Drawings and described in the Detailed Description, itwill be understood that the invention is not limited to the embodimentsdisclosed, but is intended to embrace any alternatives, modificationsand/or substitutions of parts and elements falling within the scope ofthe invention as defined by the following claims.

We claim:
 1. A constant velocity universal joint, comprising:ahelicopter rotor mast rotatable about a first axis; a yoke driven bysaid mast and rotatable about a second axis; at least three pillowblocks rigidly mounted on said yoke; carrier means for mounting at leastthree pairs of elastomeric bearings, each of said elastomeric bearingpairs comprising a drive bearing and a driven bearing joined by saidcarrier means in spaced apart relationship, each of said driven bearingsresiliently connecting said carrier means to one of said pillow blocks;means for joining said mast to said at least three drive bearings, saidjoining means mounted on said shaft for rotation therewith, said drivebearings resiliently connecting said carrier means to said joiningmeans, said bearings having a predetermined radial spring rate relativeto the first and second axes to transmit torque from the helicopterrotor mast to the yoke in a predetermined angular spring rate allowingmisalignment of said first and second axes and provide for constantvelocity rotation for said yoke; resilient means connected between saidmast and said yoke for limiting the angle of misalignment between saidfirst and second axes and for transmitting axial load and shear forcesfrom said mast to said yoke; said resilient means comprising elastomericmembers secured between said mast and said yoke, said elastomericmembers forming sections of a sphere generally centered on a pivot pointabout which said first and second axes intersect when said axes aremisaligned, said elastomeric members limiting the misalignment betweensaid mast and said yoke and exerting a control moment about said pivotpoint to realign said first and second axes; said elastomeric memberscomprising a lower spring element and an upper spring element; the lowerspring element splined for rotation with said mast below said joiningmeans; and the upper spring element splined for rotation with said mastabove said joining means.
 2. The constant velocity universal joint ofclaim 1, wherein said elastomeric bearings deform as said mast rotatessaid yoke, and wherein said carrier means has an axis of symmetry thatbisects any angle or misalignment between said first and second axes toensure constant velocity motion of said yoke.
 3. The constant velocityuniversal joint of claim 1 wherein each of said bearings furtherincludes a predetermined axial spring rate generally parallel the firstand second axes to transmit thrust from the yoke to the helicopter rotormast.